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Astrophys. Space Sci. Trans., 2, 53–61, 2006
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Astrophysics and Space Sciences
Transactions
The Sun’s journey through the local interstellar medium: the
paleoLISM and paleoheliosphere
P. C. Frisch1 and J. D. Slavin2
1 University
of Chicago, Department of Astronomy and Astrophysics, 5640 S. Ellis Ave., Chicago, IL 60637, USA
Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
2 Harvard-Smithsonian
Received: 12 April 2006 – Revised: 30 June 2006 – Accepted: 30 June 2006 – Published: 8 August 2006
Abstract. Over the recent past, the galactic environment
of the Sun has differed substantially from today. Sometime
within the past ∼ 130 000 years, and possibly as recent as
∼ 56 000 years ago, the Sun entered the tenuous tepid partially ionized interstellar material now flowing past the Sun.
Prior to that, the Sun was in the low density interior of the Local Bubble. As the Sun entered the local ISM flow, we passed
briefly through an interface region of some type. The low
column densities of the cloud now surrounding the solar system indicate that heliosphere boundary conditions will vary
from opacity considerations alone as the Sun moves through
the cloud. These variations in the interstellar material surrounding the Sun affected the paleoheliosphere.
1 Introduction
The boundary conditions of the heliosphere at any given
point in its history are set by the interstellar cloud that happened to surround the Solar System at that time. Our variable
galactic environment affects the physical properties of the
heliosphere, and the fluxes of galactic cosmic rays and interstellar byproducts reaching the Earth. The work of Hans Fahr
has, from the earliest until now, provided a strong foundation
for our understanding of the effect of the ISM on the heliosphere and on Earth (e.g. Fahr, 1968, 1974; Ripken and Fahr,
1983; Scherer and Fahr, 2003; Yeghikyan and Fahr, 2004).
The properties of both astrospheres and the heliosphere are
highly responsive to the boundary conditions supplied by the
interstellar medium and the interstellar radiation field (Fahr,
1978; Frisch, 1993, 1997; Zank and Frisch, 1999; Scherer
et al., 2002; Florinski et al., 2003b; Mueller et al., 2006;
Frisch, 2006). The space motion of the Sun, when compared
to the column densities of interstellar material (ISM) towards
Correspondence to: P. Frisch
([email protected])
stars near the Sun, indicates that sometime in the late Quaternary the Sun, which has been moving through the very low
density region known as the Local Bubble, encountered the
cluster of local interstellar clouds (CLIC) flowing away from
the direction of the Scorpius-Centaurus Association (Frisch,
1981, 1997; Frisch and York, 1986; Frisch and Slavin, 2006,
FS06). Mediating the interaction between the very low density Local Bubble and the tepid CLIC will be a thin interface
of some type (Slavin, 1989; Slavin and Frisch, 2002).
Regardless of the Local Bubble plasma pressure, the large
contrast (> 103 ) between neutral ISM densities in the Local Bubble versus the CLIC must have generated significant changes in the heliosphere, because pickup ions and
anomalous cosmic rays are processed interstellar neutrals.
The well-known anticorrelation between galactic cosmic ray
fluxes at Earth and the solar magnetic activity cycle, mediated by the heliosphere, suggests that the transition between
the Local Bubble cavity and CLIC altered both the heliosphere and the galactic cosmic ray flux at Earth, with possible implications for the terrestrial climate. Heliosphere
models have now been constructed for a range of interstellar boundary conditions (e.g. Mueller et al., 2006; Florinski
et al., 2003a; Scherer, 2000; Yeghikyan and Fahr, 2004). The
models show that the ISM surrounding the Sun during the geological past (or the “paleoLISM”) did affect the heliosphere
during the geological past (or the “paleoheliosphere”), and
therefore also the galactic cosmic ray flux at Earth.
In this paper, we postulate a geometric model for the
CLIC, supplemented by equilibrium models that provide the
cloud density. The entry epoch of the Sun into the CLIC (also
known as the Local Fluff) depends on the relative Sun-CLIC
velocities and the distribution of the CLIC gas. To estimate
this transition epoch, the distribution of CLIC gas is derived
from a simple model of the cloud morphology using data
for Ho and Do towards nearby stars (Sect. 2.1), combined
with the CLIC density found from photoionization equilibrium models of nearby ISM (Sect. 2.2). The entry of the Sun
Published by Copernicus GmbH on behalf of the Arbeitsgemeinschaft Extraterrestrische Forschung e.V.
54
P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
Table 1. HC and LSR Velocities of the Sun, CLIC, and LIC.
HC:
V (km/s)
`,b
LSRStd :
V (km/s)
`,b
LSRHip :
V (km/s)
`,b
Sun
Motion
Upwind
CLIC
Upwind
LIC
–
–
–28.1±4.6
12.4◦ , 11.6◦
–26.3±0.4
3.3◦ , 15.9◦
19.5
–19.4
–20.7
56◦ , 23◦
331.0◦ ,–5.1◦
317.8◦ , –0.5◦
13.4
–17.0
–15.7
27.7◦ , 32.4◦
2.3◦ , –5.1◦
346.0◦ , 0.1◦
Note: The Standard solar motion, LSRStd , corresponds to a velocity
of 19.5 km/s towards `=56◦ , b=23◦ . The Hipparcos solar motion,
LSRHip , corresponds to a velocity of 13.4 km/s towards `=27.7◦ ,
b=32.4◦ (Dehnen and Binney, 1998).
into the CLIC is then found from the cloudlet velocities in
the downwind direction (Sect. 2.1). Prior to encountering the
CLIC, the Sun was in the very low density gas of the Local
Bubble interior for several million years (Sect. 3). The limitations of the simple assumptions underlying these estimates
are discussed briefly in Sect. 4.
2 Sun passage into very local ISM
The epoch when the Sun first encountered the CLIC gas can
be estimated from data on ISM column densities and velocities towards nearby stars, combined with theoretical models that provide the average cloud density. These photoionization models provide a second important result about the
paleoheliosphere, by showing clearly that the boundary conditions of the heliosphere, in particular the ionization level
of hydrogen and the electron density, vary from radiative
transfer effects alone as the Sun traverses low opacity ISM
(Sect. 2.2, Slavin and Frisch, 2002, SF02,SF06).
2.1
Local ISM data and distribution
The Sun has recently entered the CLIC, which itself is inhomogeneous. To estimate the entry date, we map the CLIC
gas by assuming that the distances to the CLIC edges are
given by N (Ho )/hn(H◦ )i, where N (Ho ) are column densites
towards nearby stars. The average space density hn(H◦ )i is
provided by radiative transfer models (Sect. 2.2).
CLIC dynamics indicate an origin related to a superbubble caused by star evolution in the Scorpius-Ophiuchus Association (Frisch, 1981). The bulk flow velocity vector of
the CLIC gas past the Sun is ∼ −28 ± 4.6 km/s, from the direction `,b ∼ 12◦ ,12◦ in heliocentric coordinates (HC, Frisch
et al., 2002). This vector is based on absorption components
Astrophys. Space Sci. Trans., 2, 53–61, 2006
towards ∼ 60 stars, obtained at resolutions of 0.3–3.0 km/s.
Most of the Hyades stars were excluded from the star sample underlying this vector, because ISM with a poorly defined relationship to the CLIC is found inside of this cluster (Redfield and Linsky, 2001). The LIC is the best understood member of the CLIC, with a precisely known velocity based on Ulysses He◦ data of − 26.3 ± 0.4 km/s from
`,b = 3.3◦ , 15.9◦ (HC, Witte, 2004). The exact value for the
CLIC bulk flow depends on the underlying star sample, since
the flow gradually decelerates towards the downwind direction (FS06). Examples of the deceleration are the LIC, found
in the downwind direction at + 26 km/s, and the Apex cloud,
found at –35 km/s in the upwind direction (HC).
The velocity vectors of the CLIC, LIC, and the Sun are
listed in Table 1 in both HC coordinates and the local standard of rest (LSR) velocity frame, for both the Standard and
Hipparcos-based LSR frame. The uncertainty in the LSR
occurs because the age distributions of the star samples underlying these two LSR vectors were different (Dehnen and
Binney, 1998; Mihalas and Binney, 1981).
Data on N (Ho ) are drawn from observations of Do and
towards nearby cool and hot stars (Wood et al., 2005;
Redfield and Linsky, 2004a; Frisch et al., 2002), white dwarf
stars (Wolff et al., 1999; Lehner et al., 2003; Vallerga, 1996;
Oliveira et al., 2003; Hébrard and Moos, 2003; Wood et al.,
2002; Lemoine et al., 2002; Kruk et al., 2002; Frisch, 1995),
and nearby stars with observations of interstellar Ca+ (e.g.
Frisch et al., 2002; Crawford et al., 1998; Crawford and
Dunkin, 1995, and references therein). The column densities
of all velocity components towards a star are summed to obtain the total N (Ho ) towards the CLIC surface. For stars with
Do data, a conversion factor of N(Ho )/N(Do ) = 1.5 × 10−5 is
used. The total Ho column densities to the CLIC surface are
in the range N (Ho ) = 0.3 × 1018 to ∼ 1019 cm−2 .
Ho
Optical Ca+ data are also used, however the ratio
N(Ca+ )/N (Ho ) is highly variable. In cold clouds ∼ 99.7% of
the Ca is depleted onto dust grains. Both depletion and ionization affect Ca+ in warm clouds such as the CLIC; for example Ca++ /Ca+ > 1 if T > 4 000 K and n(e) < 0.13 cm−3
(e.g. Welty et al., 1996). The ratio N (Ho )/N (Ca+ ) = 10−8 is
used, based on the three absorption components observed in
both Ca+ and Ho towards α Aql (d = 5 pc, `,b∼ 48◦ ,–9◦ , e.g.
Frisch et al., 2002; Redfield and Linsky, 2004a; Ferlet et al.,
1986).
Looking only at stars within 10 pc, average values of
hN (Ho )i ∼ 1018 cm−2 and hn(H◦ )i ∼ 0.07 cm−3 are found.
Based on temperature and turbulence data in Redfield and
Linsky (2004b), objects within 10 pc show a temperature and turbulence range of T = 1 700 − 12 600 K and
ξ = 0 − 5.5 km/s, with mean values 6 740 ± 2 800 K and
ξ = 1.9 ± 1.0 km/s. The ξ variable represents deviations
from a Maxwellian velocity distribution for the atoms contributing to the absorption line components, and as such is a
mock turbulence that includes unresolved clouds.
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P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
Fig. 1. The distances to the edge of the CLIC, projected onto the
galactic plane, as calculated from N(Ho )/hn(H◦ )i for stars near the
Sun. Distances are based on observations of Ho (dots), Do (dots),
and Ca+ (crosses) (see text). The directions ` = 0◦ , 90◦ , 180◦ , and
270◦ are labeled. The arrows directed to the left show the CLIC motion through the Local Standard of Rest (LSR) based on the standard
(solid) and Hipparcos (dashed) solar apex motions, while the arrows
to the right show the motion of the Sun through the LSR based on
the Hipparcos and Standard apex values (Table 1).
The range of temperatures inferred for the CLIC, combined with the macroturbulence of ± 4.6 km/s, show that the
CLIC is inhomogeneous and that the boundary conditions of
the heliosphere will vary during the next thousands of years
as the Sun traverses the CLIC.
The CLIC is fully contained in the nearest ∼ 35 pc of
space. The distances to the edges of the CLIC are shown
in Figs. 1 and 2. These figures are constructed from the data
listed above, and assuming hn(H◦ )i = 0.17 cm−3 (discussed
in Sect. 2.2). This ISM distribution is assumed to contain
no gaps.
An extended dilute H II region found towards λ Sco
(York, 1983) may contribute to the excess cloud length towards the galactic center that is indicated by Ca+ data.
Highly ionized gas at CLIC velocities is found towards
both λ Sco (`,b = 352◦ ,–2◦ ) and HD 149499B (d = 37 pc,
`,b = 330◦ ,–7◦ ), where for instance N+ /N◦ = 1.9 (Lehner
et al., 2003).
2.2
Boundary condition variations from cloud opacity
An important influence on the heliosphere while the Sun
traverses the low opacity CLIC is the change in heliowww.astrophys-space-sci-trans.net/2/53/2006/
55
Fig. 2. Same as Fig. 1, except that the projection is on a meridian
plane perpendicular to the galactic plane.
spheric boundary conditions caused by ionization variations
due to the attenuation of photons with energy E > 13.6 eV
(the ionization edge of Ho ). These variations are shown
by our radiative transfer (RT) models (Slavin and Frisch,
2002)1 . The CLIC is partially opaque to H-ionizing photons
(λ < 912 Å) and nearly transparent to He-ionizing photons
(λ < 504 Å). A cloud optical depth of τ ∼ 1 is achieved for
logN (Ho ) ∼ 17.2 cm−2 and ∼ 17.7 cm−2 , respectively, at the
Ho and He◦ ionization edges.
A series of models have been constructed to study these
opacity effects (SF02, SF06). These models employ the
CLOUDY radiative transfer code, which incorporates a wide
range of physical processes to model a cloud under the conditions of photoionization equilibrium (Ferland et al., 1998).
The models are constrained by observations of the CLIC
towards CMa (Gry and Jenkins, 2001), observations of
pickup ions, anomalous cosmic rays, and n(He◦ ) inside of the
solar system, and interstellar radiation field data and models.
The radiation field is based on radiation sources affecting the
solar environment (see SF02), and extends out to soft X-ray
energies (Sect. 3.2). Figure 3 summarizes the variations in
neutral densities that are obtained for equilibrium calculations of low density ISM similar to the LIC (see SF02 and
1 By “radiative transfer effects”, we mean that the spectrum of
the radiation field, including both point source and diffuse contributions, is substantially modified for energies E > 13.6 eV as the
radiation penetrates more deeply into the cloud. This is the case for
the CLIC gas, as shown by the relative opacities of the cloud to Hversus He- ionizing photons.
Astrophys. Space Sci. Trans., 2, 53–61, 2006
56
P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
Fig. 3. The densities of Ho and He◦ , as predicted by radiative
transfer models of the CLIC, are shown for a range of equilibrium
conditions based on models in SF02 and SF06. The symbols give
the model parameters: symbol colors indicate the magnetic field
strength in the cloud interface; symbol fill gives gives the assumed
HI column density; symbol shape gives the assumed temperature of
the Local Bubble plasma. The stars are special parameter sets which
do not fall on the grid of model parameters, but rather are chosen to
better match the data. The range of current values for n(He◦ ), based
on Ulysses and pickup ion data, are shown as vertical lines (Möbius
et al., 2004). Although a range of n(H◦ ) values are consistent with
n(He◦ ), the radiative transfer models providing the best agreement
with ISM observations both inside of the heliosphere and towards
nearby stars give n(H◦ ) = 0.18 − 0.21 cm−3 .
SF06). The best of these models (models 2 and 8 in SF02)
give n(H◦ ) = 0.18 – 0.21 cm−3 and n(H+ ) = 0.1 cm−3 at the
solar location. These model results that best predict the observed gas densities, for a study still in progress, give a mean
value of hn(H◦ )i ∼ 0.17 cm−3 for the sightline towards the
downwind cloud surface.
Figure 4 shows the effects of cloud opacity. The ionization
levels of H, O, and N, which are coupled by charge exchange,
decrease by ≥20% from the cloud exterior to the solar location.2 In contrast, He and Ne ionizations, which require photons more energetic by > 50%, vary little. Guesstimates indicate that for ∼ 50% filtration of Ho , converting 20% of the
H from Ho to H+ would raise the H pressure confining the
paleoheliosphere at the cloud surface on the order of ∼ 10%,
as compared to the present-day value. The timescales over
which these variations occur depend on the assumed cloud
shape, and may be as short as thousands of years.
Ionization variations that are five times larger than those
we have modeled are observed in the very local ISM.
Warm partially ionized material (WPIM, T > 5 000 K) such
as the CLIC is widespread near the Sun, and is sampled
2 The small decrease in N◦ /Ho at cloud edge results from a high
N◦ photoionization cross-section.
Astrophys. Space Sci. Trans., 2, 53–61, 2006
Fig. 4.
Variation of neutral densities, due to radiative transfer effects between the Sun and cloud surface, for Model 2 from
SF02. Shown are variations in neutral densities between the Sun
(N(Ho ) = 6.5 × 1017 cm−2 ) and cloud surface (N(Ho ) = 0) for Ho ,
He◦ , Ne◦ , O◦ , and N◦ . At the heliopause, n(He◦ ) ∼ 0.015 cm−3 ,
n(H◦ ) ∼ 0.21 cm−3 , n(e) ∼ 0.1 cm−3 . The cloud surface is at the
left, and the solar location is at the right of the figure.
by data on N+ and N◦ from Copernicus and FUSE. Values of N (N+ )/N (N◦ ) ∼ 0.4 − 2.0 are typical for low density
sightlines, N (Ho ) < 1019 cm−2 (e.g. Rogerson et al., 1973;
Oliveira et al., 2003; Lehner et al., 2003). The best CLIC
models give n(N+ )/n(N◦ ) ∼ 0.76 at the Sun (nos. 2 and 8 in
SF02). Consequently, as the CLIC sweeps past the Sun variations larger than 10% in the heliosphere may be caused by
variable interstellar ionizations. The H+ gas towards λ Sco
and HD 149499B, 37 pc away in the upwind direction, is an
example of an ionized cloud that could engulf the Sun in the
next million years.
2.3
Solar encounter with the CLIC
When did the Sun enter the CLIC? With the exception of
the LIC, only the Doppler-shifted radial components of cloud
motions are observed. Hence only approximate estimates of
the epoch that the Sun made the transition from the very low
density Local Bubble to the CLIC are possible. We get an
averaged value for the time of encounter by calculating the
distance to the CLIC edge for all stars within 50 pc, and between ` = 170◦ ± 30◦ and |b| < 30◦ , and comparing this distance and the HC ISM velocity towards each star. The distance to the cloud edge is given by N(Ho )/hn(H◦ )i, where
hn(H◦ )i = 0.17 cm−3 . Nine stars in Figs. 1 and 2 fall in this
interval. The average distance to the cloud edges for these
stars is 2.8 pc (range 1.2–3.8 pc), and the average entry time
of the Sun into the CLIC is 120 000 years ago (range 56 000–
200 000 years ago). Restricting the downwind stars to those
within 30 parsecs yields four stars, and does not significantly
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P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
200
100
0
-100
-200
-200
-100
0
100
200
the thermal pressure, PTH . Magnetic field strengths of
B ∼ 3 µG are required in the LIC to balance thermal pressures of PTH ∼ 2 500 cm−3 K, as expected from the best photoionization models with n(e) ∼ 0.1 cm−3 , H+ ∼ 0.1 cm−3 ,
n(H◦ ) ∼ 0.2 cm−3 , and T = 6 300 K. Weaker fields would indicate compressible turbulence and possibly gaps between
the cloudlets. In such a case, the entry of the Sun into the
CLIC may have occurred earlier than stated above.
The LIC is observed towards both α CMa, at 2.7 pc, and
CMa (towards ` ∼ 230◦ , b ∼ –10◦ , Hebrard et al., 1999;
Gry and Jenkins, 2001). The LIC N (Ho ) towards α CMa
has been used to set constraints on the entry date of the Sun
into the LIC (Frisch, 1994; Mueller et al., 2006; Frisch and
Slavin, 2006). Although the LIC velocity is well known, the
cloud structure is uncertain, and we can only state that the
Sun first encountered the LIC within the past 40 000 years,
and possibly within the past 3 000 years, based on the earlier
discussions.
3
Fig. 5. The distribution of ISM is shown for non-variable stars
within ± 25◦ of a meridian slice perpendicular to the galactic plane
and aligned with the solar apex motion through the LSR. The solar apex motion is plotted as an arrow. The X-axis points towards a galactic longitude of ` = 40◦ , and the Z-axis points towards
the North Galactic Pole. The filled symbols show sightlines with
E(B − V ) > 0.06 mag (or logN(H) > 20.48, cm−2 ), and the x’s
show sightlines with E(B − V ) < 0.02 mag (or logN(H) < 19.93,
cm−2 ). Open squares have E(B − V ) = 0.02–0.06 mag. Large open
circles display the positions of nearby dust clouds (see text). These
reddening values are determined from photometric data in the Hipparcos catalog (Perryman, 1997). Data points are smoothed over
± 13◦ , and incorporate all stars with overlapping distances (once
uncertainties are included). The asterisks indicate the positions of
the stars CMa and β CMa, which are located in a well-known spatial region with low interstellar densities (Frisch and York, 1983),
but which are not included in the plotted star sample because the
Hipparcos catalog flags them as variable.
change these entry time values. The star closest to the HC
downwind direction of the LIC (`,b ∼ 183◦ ,–16◦ ) is χ 1 Ori
(HD 39587, at 8.7 pc). The HC cloud velocity of 21.6 km/s
towards this star (Redfield and Linsky, 2004a) indicates an
entry date into the CLIC of ∼ 56,000 years ago. We conclude
that for no gaps in the ISM distribution within the CLIC, so
that hn(H◦ )i ∼ 0.17 cm−3 is valid, then the Sun entered the
CLIC within the past ∼ 130 000 ± 70 000 years, and possibly within the past 56 000 years (consistent with our earlier
estimates, Frisch, 1997; Frisch and Slavin, 2006).
If the velocity dispersion of the CLIC clouds represents
turbulence, then the assumption of no gaps implies incompressible turbulence. A magnetized partially ionized tenuous plasma will have incompressible turbulence if the magnetic pressure, PB , is on the order of, or greater than,
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57
The Sun in the Local Bubble cavity
Prior to crossing paths with the CLIC, the Sun traveled
through the Local Bubble (LB) for several million years
(Frisch and York, 1986). The path of the Sun through the
Local Bubble is reconstructed below from reddening data
(Sect. 3.1). The LB interior formed one type of paleoLISM,
and the limits on the plasma properties of the Local Bubble
interior are discussed in Sect. 3.2.
3.1 Local Bubble
The exact dimensions and structure of the Local Bubble depend on the ISM component that is sampled; we use the optical reddening properties of interstellar dust grains here, based
on photometric data in the Hipparcos catalog (Perryman,
1997), and find a radius of ∼ 60–100 pc. Earlier maps of
the Local Bubble ISM distribution based on reddening data
that traces interstellar dust (Lucke, 1978; Perry and Johnston, 1982; Perry and Christodoulou, 1996; Vergely et al.,
1997), dust and magnetic fields traced by polarization data
(Mathewson and Ford, 1970; Leroy, 1999), and gas traced by
Ho and Nao (Frisch and York, 1983; Paresce, 1984; Vergely
et al., 2001; Lallement et al., 2003) yield similar conclusions
about the bubble dimensions, although details of the cavity
topology depend on data sensitivity to low column density
ISM. Figure 5 shows the motion of the Sun through the Local Bubble, for a plane passing through a meridian, perpendicular to the galactic plane, and aligned with an axis coinciding with the solar apex motion, extending from ` = 40◦ to
220◦ . Stars with longitudes within ± 25◦ of this meridian
slice are plotted. Cleaned and averaged photometric and astrometric data for O, B, and A stars in the Hipparcos catalog
are used, with variable stars and stars with poorly defined
spectral types omitted.
Astrophys. Space Sci. Trans., 2, 53–61, 2006
58
P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
In tenuous (“intercloud”) material,
color excess,
E(B − V ),
and N (H) are related by
logN (H)/E(B − V ) = 21.70 cm−2 mag−1 (Bohlin et al.,
1978). The CLIC reddening is E(B − V ) < 0.002 mag, and
would not appear on this figure. The large circles show dust
clouds from Dutra and Bica (2002). It is evident that for a
solar LSR velocity of 13 − 20 pc Myrs−1 , the Sun has been
within the very low density Local Bubble for over 3 Myrs.
Although CLIC column densities ( < 1019 cm−2 ) are not
traced by typical E(B − V ) data, ultraviolet absorption lines
do not show any neutral ISM in the anti-apex direction,
` ∼ 220◦ , at the N(Ho ) ∼ 1017 cm−2 level between ∼ 5 and
∼ 100 pc (Frisch and York, 1983, FS06).
3.2
Local Bubble interior
We now discuss the properties of the paleolism when the Sun
was in the very low density LB interior. The nature and origin of the Local Bubble is the subject of ongoing debate. We
expect that this volume is presently filled with a very low
density ionized plasma that provides pressure support for the
cavity. Observations of the diffuse soft X-ray background
(SXRB) led to models of the Local Bubble filled with hot,
high pressure gas with a temperature of ∼ 106 K and pressure of P /kB ∼ 104 cm−3 K (kB is the Boltzman constant),
although ROSAT has shown that substantial amounts of the
SXRB at high latitude arises beyond the boundaries of the
Local Bubble (McCammon et al., 1983; Kuntz and Snowden, 2000). An effective plasma temperature near 106 K
is indicated for a plasma in collisional ionization equilibrium (CIE). Disregarding the problems with models of the
emission spectrum, under the assumption that there is a CIE
hot plasma filling the LB, the implied density is roughly
5 × 10−3 cm−3 and pressure of ∼ P /kB = 104 cm−3 K.
There are several implications of the SXRB attributed to
the Local Bubble. First, the intensity of the background implies a thermal pressure that is substantially higher than that
of the CLIC. This mismatch may be fixed by a CLIC magnetic field of B ∼ 4 µG. Second, a potentially substantial
contribution (< 30% in most directions) to the SXRB may
arise from charge transfer between solar wind ions such as
O+7 and neutral H or He, either in the interstellar wind or
geocorona (Cravens, 2000; Cravens et al., 2001) . This does
not affect the photoionization calculations described above
directly, however, even if the heliospheric and geocoronal
soft X-ray emission is at the upper end of current estimates.
That is because the heliospheric and geocoronal soft X-ray
emission is substantially harder than the emission that is
directly responsible for LIC ionization (∼ 13.6 − 54.4 eV).
There is an indirect effect, though, insofar as the heliospheric
X-ray emission affects our understanding of the source of the
SXRB, which in turn has implications for the lower energy
emission and the cloud interface.
This low density hot LB plasma constituted the solar
galactic environment for several million years. The LB
Astrophys. Space Sci. Trans., 2, 53–61, 2006
plasma properties have probably been constant since then,
because cooling times for low density plasmas are over
≈ 100 Myrs. Nevertheless, the LB properties are not constant. Interstellar shocks move through space at velocities over 100 km/s (∼ 100 pc Myrs−1 ), quickly traversing low
density ISM. Also, 25% of the ISM mass is contained in
clouds traveling faster than 10 km/s through the LSR (based
on Ho 21-cm data in Heiles and Troland, 2003). Therefore,
over the past several Myrs some dilution of the plasma must
have occurred, particularly near the LB boundaries. The
CLIC, with VLSR ∼ 18 km/s may be an example.
The paleoheliosphere formed by the interactions between
the very low density plasma of the Local Bubble void and the
solar wind has been modeled by Mueller et al. (2006). The
resulting heliosphere dimensions are similar to the present
heliosphere, but the heliosheath is thicker and pickup ions
and other derivatives of interstellar neutrals are absent. These
changes have opposite effects on the modulation of galactic
cosmic ray fluxes at the Earth.
3.3 Interface between Local Bubble Plasma and CLIC
One additional consequence of the presence of the Local
Bubble plasma is that a transition region must exist between
the tepid CLIC gas and the hot gas. This transition ISM
constituted the paleoLISM for a very brief period of time.
Models of the interface as a thermally conductive interface
indicate a thin region of intermediate temperature gas with
the temperature changing by an order of magnitude over a
distance of ∼ 1 000 AU and more than doubling in the space
of 100 AU (Slavin, 1989, SF02). The temperature profile of
such a region is very steep near the cloud and flattens farther
out as the temperature approaches that of the hot gas. The
density profile is similarly shaped, though inverted because
the thermal pressure remains nearly constant in the interface.
The density increases into the cloud by an order of magnitude
over a distance of roughly 1 000 AU. Over this same distance
the ionization is decreasing from nearly complete ionization
of H to the ∼ 30% of the cloud interior.
The cloud gas is evaporated by the influx of heat and flows
off the cloud, accelerated by a gentle pressure gradient created by the thermal conduction. If such a profile exists at the
edge of the CLIC gas, the Sun would have traversed from
gas at T ∼ 105 K to ∼ 7 000 K in a period of about 500 years.
This would result in a sudden change in the heliospheric
boundary conditions that could be very disruptive and result
in conditions far from any equilibrium configuration.
An alternative possibility for the boundary is that of a turbulent mixing layer (Slavin et al., 1993). This would be the
case if there is a substantial velocity difference between the
CLIC gas and that of the hot gas. Even relative velocities on
the order of the sound speed in the hot gas or less could be
very disruptive to the cooler clouds and could lead to an interface in which the cool gas is being entrained in the hotter
gas, mixed and then cooled. Such an interface is similar in
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P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
some ways to the evaporative boundary described above, but
depends on hydrodynamic instabilities to create the mixing
of the cloud gas into the hot bubble gas. The crossing of this
type of interface could also be very disruptive of the heliosphere with the likelihood of small scale condensations and
velocity fluctuations as well as sudden variations in ionization in the matter incident on the Solar System.
4 Discussion
Simple assumptions are necessary to estimate the eras of
the solar transition between galactic environments, although
we know that in general the ISM contains a wide range of
cloud types and shows coherent structures ranging in size
from < 1 pc (e.g. the LIC) to > 100 pc (e.g. Loop I). Another
limitation, following from the spectral analysis techniques
used to acquire data, is that the identification of an interstellar “cloud” rests on a median velocity determined from the
Doppler spread of atomic velocities, which is highly sensitive to the resolution of the instrument conducting the observations (Welty et al., 1994).
Over the ordinary long sightlines of the ISM, e.g.
> 100 pc, similar simple assumptions are used for clouds that
are obviously blended in velocity. The more confined CLIC
gas within ∼ 10 pc allows higher levels of accuracy. Howevever, although the electron density diagnostics Mgo /Mg+
and C+∗ /C+ show consistent values of n(e) ∼ 0.1 cm−3 towards several stars sampling the CLIC, the neutral densities
must be reconstructed from radiative transfer models because
C0 fine-structure data are unavailable (e.g. Jenkins and Tripp,
2001). Model results indicate consistently that the CLIC
does not fill the sightline towards any nearby star, with filling factors of ∼ 0.40 in the galactic center hemisphere and
∼ 0.26 in the anti-center hemisphere for n(H◦ ) ∼ 0.2 cm−3
(Frisch and Slavin, 2006). Except for observations of Si++
towards local stars (e.g. Gry and Jenkins, 2001) and pickup
ion Ne, radiative transfer models successfully reproduce densities of ISM towards CMa and within the solar system.
Therefore, except for possible hidden dense small clumps of
gas, we expect that a cloud density of n(H◦ ) ∼ 0.2 cm−3 is a
reasonable value. Less certain is whether there are gaps in
the distribution of the nearest ISM, as would be expected for
filamentary LIC gas (Frisch, 1994).
Nevertheless, our understanding of the nearest ISM is not
complete, and in particular the origin of the observed macroturbulence indicated by the velocity data (Frisch et al., 2002)
and the possibility of denser material in the upwind direction (Frisch, 2003) indicate much of the local ISM remains
a mystery. The LIC is our best understood cloud, yet distinctly different estimates of the date of the Sun’s entry in the
LIC are found depending on which subset of data are used
to define an assumed filamentary LIC shape (resulting in entry eras of less than ∼ 10 000 years ago and up to ∼ 40 000
years ago, Frisch, 1994; Mueller et al., 2006). Our analysis
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59
here of the paleoLISM provides a starting point for future
studies based on improved data and a deeper understanding
of turbulence and small-scale structure in the CLIC.
The contribution to the soft X-ray background (SXRB)
radiation field at the Sun due to charge exchange between
the solar-wind and interstellar neutrals was discussed in
Sect. 3.2. The primary effect of this emission will be on
the properties of the cloud interface, while the overall soft
X-ray flux will continue to be dominated by the strong emission from the Loop I supernova remnant and attenuated extragalactic sources.
5
Conclusions
The primary conclusion of this paper is that, over the past
several million years, both the galactic environment of the
Sun and the heliosphere have been significantly different than
they are today. Observational data combined with theoretical studies can be used to reconstruct the three-dimensional
distribution of nearby ISM, and predict the times the Sun
transitioned between different environments. If we assume a continuously distributed local ISM, within the past
∼ 130 000 ± 70 000 years, and possibly as recent as ∼ 56 000
years ago, the Sun entered low density partially ionized ISM
flowing away from the direction of the Scorpius-Centaurus
Association. Sometime within the past ∼ 40 000 years the
Sun entered the cloud now surrounding the solar system,
the LIC. These estimates rely on topologically simple models of the cluster of local interstellar clouds (CLIC) flowing
past the Sun; more elaborate models are discussed elsewhere
(Frisch, 1994; Gry, 1996; Mueller et al., 2006, FS06). As the
Sun moves through this complex of local interstellar clouds,
the boundary conditions of the heliosphere should change by
substantial amounts due to changes in cloud temperature, velocity, and opacity-driven variations in the ionization of the
surrounding ISM. Prior to that, the Sun was in the low density plasma of the Local Bubble cavity. Between the Local
Bubble cavity and the CLIC, the Sun briefly (∼ 500 years)
passed through an interface region of some type.
These estimates of the entry date of the Sun into the CLIC
and LIC are based on current data and models. Future veryhigh resolution observations in the 1 000–3 000 Å spectral interval, for a spatially dense sample of nearby stars, are required to reconstruct the distribution, kinematics, and properties of the CLIC, and reduce the uncertainties in these estimates. When that happens, we can anticipate that the early
work of Hans Fahr will have yielded grand results, as we
finally understand the close relationship between the paleoheliosphere and paleoLISM.
Acknowledgements. The authors acknowledge support for this
research by NASA grants NAG5-11005, NAG5-13107, NAG513558, and NNG05GD36G.
Astrophys. Space Sci. Trans., 2, 53–61, 2006
60
P. C. Frisch and J. D. Slavin: Variations in the solar galactic environment
Edited by: H.-J. Fahr
Reviewed by: A. Yeghikyan and another referee
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